Discovering ways to minimize the resistance-capacitance (RC) delay of metal interconnects is a primary pursuit within the high performance integrated circuit manufacturing industry. The RC delay is the time it takes a signal to propagate through a metal interconnect as a result of the resistance of the metal lines and the capacitance of the surrounding dielectric. Reducing the RC delay increases the speed of the signal propagation, thereby improving the performance of a device. One approach to reducing RC delay is by using lower resistance metals; for example, copper instead of aluminum. Another approach to reducing RC delay is by reducing the capacitance of the surrounding dielectric by using a material with a low dielectric constant, k.
The dielectric constant, k, is an intrinsic property of a material, which determines the electrostatic energy that can be stored within the material. The numerical value k is defined relative to a vacuum, for which k=1, exactly. In IC applications, “low-k” dielectrics are conventionally defined as those materials that have k<˜4. This low-k definition comes from the k value of SiO2, the traditional primary IC dielectric material, which has k≈4. Since current technologies involving smaller features sizes are requiring k<4, efforts have been focused on finding materials with lower k than that of SiO2.
One widely used means of obtaining a low-k dielectric material is by incorporating carbon or hydrocarbons into a SiO2 material. Since most hydrocarbons have a lower k than SiO2, the resulting carbon-incorporated material has an overall lower k than pure SiO2—typically between 3.1 and 2.5, depending on the amount of carbon added and the type of organic precursor that is used. However, as IC features continue to decrease to smaller and smaller sizes, there is a drive to find materials with even lower k than can be obtained by using the carbon-incorporated SiO2 alone.
Currently, the only known way to obtain a usable dielectric with k<2.5 is by using techniques to produce pores or voids in a dielectric material with a dielectric constant k≈2.5 to 4. Since the dielectric constant for air is only slightly higher than that of a vacuum, the resulting porous dielectric will have a significantly decreased k. A common technique for fabricating porous dielectric materials involves the formation of a composite film consisting of two components: a porogen (an organic material, typically a polymer) and a silicon-based dielectric. Once the composite film is formed onto the substrate, the organic porogen component is removed, leaving a porous silicon-based dielectric matrix.
There are significant challenges, however, associated with forming a porous dielectric material, particularly in the porogen removal process. A standard technique for porogen removal involves heating the wafer for several hours to thermally degrade and remove the porogen. Another technique involves exposing the wafer to a plasma treatment while heating the wafer. This porogen removal process creates “dangling bonds” (unsaturated SiO- or Si-groups) within the silicon-based dielectric matrix, which when exposed to ambient conditions, will react with moisture to create hydroxyl groups. These hydroxyl groups will in turn adsorb more moisture from the ambient to add water to the silicon-based dielectric matrix. Unfortunately, since water has a dielectric constant of about 80, this significantly increases the overall dielectric constant for the film.
What are needed therefore are improved methods for forming low k dielectric materials for integrated circuits.